function largestOfFour(arr) {
    return arr.map(
        function (val) {
            return val.reduce(
                function (prev, cur) {
                    return prev > cur ? prev : cur;
                });
        });
}

console.log(largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]));
